1. Field of the Invention
The present invention relates to an image recording method for recording color images on a recording medium by using a plurality of colored inks that have different densities of the same color, to express a specific basic color.
2. Description of Related Art
In recent years, color printers have come into broad distribution as image recording devices for recording computer-processed images in different tones and with a plurality of colors. Such color printers eject different colors of ink from a recording head. Generally, color printers reproduce images using ink of four basic colors of cyan (C), magenta (M). yellow (Y), and black (K), which will be referred to as xe2x80x9cnormal inksxe2x80x9d hereinafter.
In order to improve image quality, it is conceivable that color printers use xe2x80x9clight inkxe2x80x9d together with normal ink in order to reproduce a single specific color such as cyan (C) or magenta (M). Each light ink has a density lower than its corresponding normal ink. In other words, light ink and, normal ink are different inks that have different densities of the same basic color.
FIG. 1 represents how images can be reproduced using both normal and light inks for some basic color, such as cyan (C) or magenta (M). As shown in FIG. 1, when color data, inputted from a computer or the like, has a small tone value, then the basic color is reproduced using the light ink only. Variations in the tone value of the color data can be reproduced by changing the dot density of the light ink. Once the tone value of the color data reaches a predetermined reference tone value (xe2x80x9c100xe2x80x9d in this example), then normal ink starts being used together with light ink. As the tone value of the color data increases further from the reference tone value, the dot density of the light ink is gradually reduced, while the dot density of the normal ink is gradually increased to reproduce the desired tone of the basic color.
In order to form an image on a recording medium using light and normal inks in the manner described above, it is conceivable that a light ink conversion table and a normal ink conversion table are prepared in advance for each of the basic colors, such as cyan (C) and magenta (M), as shown in FIGS. 2(a) and 2(b).
In order to produce the light ink conversion table and the normal ink conversion table, the reference tone value, which indicates when to start using normal ink, should be first determined. It is conceivable that the reference tone value be determined in a manner described below.
A plurality of color patches are outputted onto a recording medium based on a plurality of tone values that differ from one another in stepwise increments. Then, normal ink is ejected in a predetermined dot density (for example, 1%) onto each color patch. An operator visually observes the color patches, and selects one or more allowable color patches where the normal ink does not give a highly noticeable dot xe2x80x9croughxe2x80x9d appearance. The operator then selects one color patch that has been produced by the smallest tone value among the selected one or more allowable color patches. The operator sets, as the reference tone value, the tone value of his/her finally-selected color patch.
Then, the conversion tables of FIGS. 2(a) and 2(b) are prepared to represent the input/output characteristics of FIG. 1. Based on the thus obtained conversion tables, color data inputted, from a computer or the like, will be converted into ink data for light ink and ink data for normal ink. Light ink and normal ink will be selectively ejected, based on the thus obtained ink data, to form an image on the recording medium.
It is noted, however, that the density level actually outputted onto a recording medium changes according to various printer characteristics, such as the type or model of the printer, the resolution, the type of ink used, and the type of recording medium. For example, it is assumed that dot xe2x80x9croughnessxe2x80x9d or light ink xe2x80x9cbleeding (blurring)xe2x80x9d do not appear noticeable when a basic color is reproduced by outputting light ink and normal ink under some condition onto a particular kind of recording medium. However, when printing is performed onto another kind of recording medium with different shade, different quality, or different absorbency, even if the printing is performed under the same condition, dot xe2x80x9croughnessxe2x80x9d and/or light ink xe2x80x9cbleedingxe2x80x9d will possibly appear noticeable. It therefore becomes impossible to always record images under optimum conditions.
Additionally, when reproducing a basic color using light ink and normal ink, as shown in FIG. 3, a point of inflection tends to occur in the output density level of the basic color at the reference tone level, that is, at the tone level where normal ink starts being used. At the inflection point, the first derivative of the output density level changes discontinuously. Such an inflection point can result in pseudo contours in the recorded image.
It is conceivable that, in order to suitably reproduce colors based on color data inputted from a computer or similar source, color correction and tons correction be performed onto the inputted color data using previously-prepared color correction table and tone correction table. Each of the color correction table and tone correction table is generally provided with correction values for some tone values, which are arranged at a fixed interval, rather than for all the tone values. Interpolation is therefore employed to calculate approximate correction values for tone values with no corresponding correction values. Color correction and tone correction are performed using the approximate correction values. However, precision of the interpolation process drops when the output density level for input color data includes the inflection point as described above with reference to FIG. 3. It becomes impossible to determine a suitable approximate correction value, and consequently impossible to perform suitable color correction or suitable tone correction.
It is therefore a first objective of the present invention to provide an image recording method that enables always recording images in a suitable condition, regardless of changes in printer characteristics, when expressing a basic color using different inks that have different densities of the same color.
It is a second objective of the present invention to provide an image recording method that enables performing suitable correction operation, without generation of pseudo contours in the image recorded on the recording medium, when expressing a basic color using different inks that have different densities of the same color.
In order to attain the above and other objectives, the present invention provides a method for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the method comprising the steps of: preparing a plurality of conversion tables, each for converting color data of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, each conversion table matching one of a plurality of sets of printer characteristics; selecting a set of printer characteristics, with which printing operation is desired to be performed; selecting one conversion table from the plurality of conversion tables in accordance with the selected set of printer characteristics: converting, based on the selected conversion table, the color data of each of the least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color; and performing printing operation to record a color image on a recording medium based on the converted color data with the selected set of printer characteristics.
As used with respect to the present invention, xe2x80x9cprinter characteristicsxe2x80x9d refers to various conditions that influence recorded images. Representative examples of the printer characteristics include; the type of recording medium, the type of ink used in the printer, the resolution of the printer, the model or type of the printer, which determines the unique qualities of the subject printer, and the print speed of the printer.
The printer characteristics selecting step may include the step of allowing a user to select his/her desired set of printer characteristics.
According to another aspect, the present invention provides a method for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the method comprising the steps of: receiving color data of at least one basic color: converting the color data of each of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, while performing tone correction on the color data of each of the at least one basic color; and performing printing operation to record a color image on a recording medium based on the converted color data.
After performing the tone correction on the received color data of each of the at least one basic color, the tone-corrected color data of each of the at least one basic color may be converted into the converted color data of the plurality of inks that have different densities of the corresponding color.
The method may further comprise the steps of: preparing a conversion table in advance; outputting, in accordance with the conversion table, color patches of each of the at least one basic color onto a recording medium using the plurality of inks that have different densities of the corresponding color; measuring density levels of the color patches; and preparing a tone correction table based on the measured density levels, and wherein the tone correction performing step performs the tone correction using the tone correction table, and the conversion step performs the conversion operation using the conversion table.
The tone correction table may be prepared to achieve a linear relationship between the color data of each of the at least one basic color and density levels to be obtained, the tone correction table being prepared by first producing a measurement curve that represents the density levels measured for the color patches and then by calculating a tone correction curve based on the measurement curve and on a predetermined linear line that represents the linear relationship.
Here, xe2x80x9clinear relationshipxe2x80x9d refers to when the color data of the basic colors and the measured density levels achieve a proportional relationship.
The plurality of different inks may include a light ink having a predetermined density and a normal ink having another predetermined density greater than that of the light ink. The conversion table may represent, for each of the light and normal inks, correspondence between a plurality of sets of color data and a plurality of sets of converted color data, the converted color data for the normal ink having zero values in correspondence with the color data of values smaller than a predetermined reference value. The color patches may be produced by using the conversion table to convert several sets of color data which include one set of color data of the predetermined reference value, into several sets of converted color data for each of the light and normal inks, and by producing each of the several color patches based on the corresponding set of converted color data for the light ink and on the corresponding set of converted color data for the normal ink.
A plurality of conversion tables may be prepared for a plurality of different sets of printer characteristics, a plurality of tone correction tables being prepared for the plurality of different sets of printer characteristics. In this case, the method may further comprise the steps of selecting a set of printer characteristics, with which printing operation is desired to be performed; selecting one tone correction table from the plurality of tone correction tables in accordance with the selected set of printer characteristics, the tone correction performing step performing the tone correction using the selected tone correction table; and selecting one conversion table from the plurality of conversion tables in accordance with the selected set of printer characteristics, the conversion step performing converting operation using the selected conversion table.
According to another aspect, the present invention provides an apparatus for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the apparatus comprising: a table storing unit storing a plurality of conversion tables, each for converting color data of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, each conversion table matching one of a plurality of sets of printer characteristics; a printer characteristics selection unit selecting a set of printer characteristics, with which printing operation is desired to be performed; a table selection unit selecting one conversion table from the plurality of conversion tables in accordance with the selected set of printer characteristics; a conversion unit converting, based on the selected conversion table, the color data of each of the least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color; and a printing unit performing printing operation to record a color image on a recording medium based on the converted color data with the selected set of printer characteristics.
According to another aspect, the present invention provides an apparatus for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the apparatus comprising: a receiving unit receiving color data of at least one basic color; a conversion unit converting the color data of each of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, while performing tone correction on the color data of each of the at least one basic color; and a printing unit performing printing operation to record a color image on a recording medium based on the converted color data.
The conversion unit may include: a tone correction unit performing the tone correction on the received color data of each of the at least one basic color; and a converting unit converting the tone-corrected color data of each of the at least one basic color into the converted color data of the plurality of inks that have different densities of the corresponding color.
According to another aspect, the present invention provides a data storage medium storing, in a manner readable by a computer, data of a program for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the program comprising: a program of preparing a plurality of conversion tables, each for converting color data of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, each conversion table matching one of a plurality of sets of printer characteristics; a program of selecting a set of printer characteristics, with which printing operation is desired to be performed; a program of selecting one conversion table from the plurality of conversion tables in accordance with the selected set of printer characteristics; a program of converting, based on the selected conversion table, the color data of each of the least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color; and a program of performing printing operation to record a color image on a recording medium based on the converted color data with the selected set of printer characteristics.
According to still another aspect, the present invention provides a data storage medium storing, in a manner readable by a computer, data of a program for recording a color image on a recording medium by expressing at least one basic color using a plurality of inks that have different densities of the corresponding color, the program comprising: a program of receiving color data of at least one basic color; a program of converting the color data of each of at least one basic color into converted color data of the plurality of inks that have different densities of the corresponding color, while performing tone correction on the color data of each of the at least one basic color; and a program of performing printing operation to record a color image on a recording medium based on the converted color data.